1. Field of the Invention
This invention relates to an air-fuel ratio control system for a multicylinder engine.
2. Description of the Prior Art
There has been known an air-fuel ratio control system for a multicylinder engine in which the air-fuel ratio in the intake mixture is detected through concentrations of exhaust emission components detected by an exhaust sensor, e.g., an oxygen sensor, disposed in the exhaust system of the engine, and closed loop control (feedback control) is effected on the air-fuel ratio in the intake mixture according to the detected air-fuel ratio.
In the known air-fuel ratio control system, an air-fuel ratio adjustment means is controlled according to an integration signal based on the output of the exhaust sensor in order to avoid abrupt change of the air-fuel ratio, and there is a certain time lag between the time intake mixture of a certain air-fuel ratio is fed to the combustion chamber and the time the air-fuel ratio of the intake mixture is detected by the exhaust sensor disposed in the exhaust system. Therefore, the actual air-fuel ratio cannot be converged on a set value, e.g., the stoichiometric air-fuel ratio, and hunting phenomenon occurs; that is, the actual air-fuel ratio periodically moves up and down across the set value. When such a hunting phenomenon occurs during idling, for example, fluctuation in torque can occur, and in the case of an engine having a catalytic convertor provided in the exhaust system, the exhaust emission cleaning properties of the catalytic convertor can be adversely affected.
In order to lessen the adverse influence of the hunting phenomenon in the closed loop control of the air-fuel ratio, there has been proposed an air-fuel ratio control system in which the cylinders of a multicylinder engine are divided into first and second groups, an exhaust sensor is provided in the exhaust system of the cylinders of the first group and the air-fuel ratio for the cylinders of the first group is controlled in a closed loop by an integration signal based on the output of the exhaust sensor, the air-fuel ratio for the cylinders of the second group being controlled by a signal obtained by inverting the integration signal for controlling the air-fuel ratio for the cylinders of the first group. That is, the air-fuel ratio for the second group cylinders is controlled by an integration signal which is reverse in phase to the integration signal for controlling the air-fuel ratio for the first group cylinders. See Japanese Unexamined Patent Publication No. 57(1982)-119140, for instance. In the air-fuel ratio control system, the air-fuel ratios for the first group cylinders and the second group cylinders fluctuate in opposite directions so that the fluctuations in torque due to the fluctuations in the air-fuel ratios for the first group cylinders and the second group cylinders cancel each other.
However, in the air-fuel ratio control system, the air-fuel ratio for the second group cylinders is controlled without detecting the actual air-fuel ratio on the presumption that the air-fuel ratio for the second group cylinders will fluctuate in the same manner as that for the first group cylinders. Even if the same fuel injection pulses are given to a plurality of fuel injection valves, for instance, the amount of fuel actually injected from each fuel injection valve differs from valve to valve due to errors in accuracy of the fuel system, errors in the volumetric efficiency of the fuel system and the like. Therefore, the air-fuel ratio for the second group cylinders may be controlled to a value far from the target value, thereby adversely affecting combustion in the cylinders, fuel economy and exhaust emission control.